Contest - Scattered Dispatch

Mark visited this small town just a few months ago and was surprised by how the town had changed since then. No more sleepy farmers on the fields, no children playing near the river. Instead, the closed gates sent him a clear signal that this night Mark would have to spend again somewhere under the nearest tree.

"Well, well, well, if it’s not Mark Atilius here! – he heard suddenly - You honestly couldn’t have come at a better time!"
"Commander! – Mark smiled broadly when the gates opened and he saw the old man he had met once in his adventures and who told him the story about the lost legion. Soon they were sitting in a guardroom, drinking hot tea and talking about past adventures. - What’s happening here and what can I do for you?" – Mark asked.

The old soldier frowned.

"I believe only an experienced traveler like you is able to help us here. We were informed about the forces headed to this direction, but the message itself was severe damaged. And we don’t even know what we should be ready to. – The old man gave Mark something that looked like the small pieces of parchment. – Can you help us? Did you see something like this before?"

Mark shivered. He almost immediately recognised the picture. And was not happy with what he was about to tell the old man.

Task: What is in the picture and what type of attack is headed to the town?

I would like to announce the winner of our first contest on the new forums. @Wilhelmina was the one with the correct answer. She was very specific in her answer which means she wins 30 gold for her account.